This invention relates to a method and apparatus for detecting an optical image of pattern information, such as a defect, of an object having a one or two-dimensional array of repetitive patterns including a pattern under inspection and a pattern which should be identical to the pattern under inspection, e.g., LSI wafers, masks of TFTs and reticles, or multi-layer thin or thick film wiring boards.
A first prior art described in Japanese Patent Publication No. 62-39811 is designed such that an optical image of a pattern on a semiconductor chip is received by an opto-electric transducer means, a digital image signal produced by the opto-electric transducer means is stored as a stored pattern as shown in FIG. 1A in a memory, an optical image of a pattern on the adjacent chip is received by the opto-electric transducer means, and a digital image signal produced by the opto-electric transducer means as the second detected pattern as shown in FIG. 1B is compared with the stored digital image signal thereby to detect a defect of pattern based on their difference as shown in FIG. 1C. Namely, in this prior art, the detection of a pattern defect is based on the comparison process between a digital image signal produced for a pattern by the opto-electric transducer means and a reference digital image signal.
A second prior art described in Japanese Patent Unexamined Publications Nos. 2-24539 and 4-111336 is designed such that a light beam produced by a light source (e.g., laser source) is split into two beams by means of a half mirror or the like, the resulting beams are projected onto two object sections, transmitted or reflected beams from the objects are merged with the same half mirror, and the output beam of this half mirror is detected with a sensor. The light paths for the two objects are set to have distances different by .lambda./2 or .lambda..times.(n+1/2) (where n=0, 1, 2, . . . ) so that optical interference is induced. The sensor output is subjected to differential detection for two patterns so that the detected signal has a zero level when both objects are the same pattern or it has a significant level if these patterns are different, i.e., the signal is predetermined at a defective portion.
In regard to the first prior art, when it is intended to detect a smaller defect, the digital image signal produced by the opto-electric transducer means must have a finer pixel size and, on the other hand, the detection speed is proportional to the square of the pixel size. The detection speed is determined from the number which is the detection area per pixel multiplied by the clock frequency of sensor per pixel multiplied by the number of parallel processing. Using the above expression, if the pixel size becomes smaller, the area of pixel becomes smaller in proportion to the square of the pixel size and the detection speed also falls in proportion to the square of the pixel size. Namely, this prior art which bases the judgement of defect on the detected digital image does not consider the performance of pattern detection in search of small defects without incurring the reduction of pattern detection speed.
In regard to the second prior art, it is necessary to set the difference of distances of the light paths passing through the two objects to .lambda./2 or .lambda..times.(n+1/2) accurately. For example, when the optical system intended for the reflective detection with a wavelength .lambda.of 633 mm has a 30 nm stability of distance of the two light paths, the light path distances may have errors as large as .lambda./10, and the detected light caused by these distance errors can possibly be judged erroneously as a defect. It is conceivably very difficult to attain a stability of 30 nm or less for the optical system with complete separate object stages or light paths because of the vibration of the object stages and the stability of air. The following explains in more detail.
The light intensity u.sub.1 and u.sub.2 reflected by the objects are expressed by the following expressions. EQU u.sub.1 =A.sub.1 .times.exp{i(w.times.t+.delta..sub.1)} (1) EQU u.sub.2 =A.sub.2 .times.exp{i(w.times.t+.delta..sub.2)} (2)
where A.sub.1 and A.sub.2 are amplitudes of the light, t is the time, .omega. is the frequency of the light, and .delta..sub.1 and .delta..sub.2 are phases.
In case the two light intensity u.sub.1 and u.sub.2 have a sufficient interference characteristics, and the two objects have an equal reflectivity and are the same pattern, the strength of interference I is given by the following expression. EQU I=2.times.A{1+cos(.delta..sub.1 -.delta..sub.2)} (3)
where A=A.sub.1 =A.sub.2.
If the distances to the two object stages have an error of .lambda./10 (e.g., for .lambda.=500 nm, the error is 50 nm), the term .delta..sub.1 -.delta..sub.2 is evaluated to be .pi..+-..pi./5 and then the detected light intensity I is evaluated to be 0.19A.sub.2 which is 4.7% of the maximum value 4A.sub.2. This light intensity can possibly be judged erroneously as a defect. As described above, the second prior art does not sufficiently consider the stability of the optical system in putting the technology into practice.